1. Field of the Invention
The present invention relates to an optical fiber applicable to large-capacity, high-speed WDM (Wavelength Division Multiplexing) optical transmission systems; and an optical transmission line including the same.
2. Related Background Art
A WDM optical transmission system enables large-capacity, high-speed optical communications since a plurality of signal lights (hereinafter referred to as WDM signals) in a 1.5-xcexcm wavelength band (1500 nm to 1600 nm) propagate therethrough by way of a network of optical fiber transmission lines. In general, this optical transmission system comprises an optical amplifier for optically amplifying the WDM signals together, and the like in addition to optical fiber transmission lines which are a transmission medium. In such WDM communications, various techniques are under study in order to enable further larger capacity and higher speed.
How to reduce the dispersion and dispersion slope has been an important subject for study with respect to optical transmission lines. It is because of the fact that, though being monochromatic, each signal light propagating through an optical transmission line has a predetermined bandwidth, whereby the signal light sent out from a transmitting station may deform its waveform when reaching a receiving station by way of the optical transmission line, thus deteriorating its reception.
For securing the quality of an optical transmission line, it is desirable that the dispersion of the optical transmission line in its signal light wavelength band be as small as possible. For realizing larger-capacity communications, on the other hand, it is necessary for the optical transmission line to suppress the dispersion in a wavelength band as wide as possible, and it is desirable that the dispersion slope of the optical transmission line be as small as possible. Therefore, dispersion-flattened optical fibers in which both the dispersion and dispersion slope are substantially zero in the 1.5-xcexcm wavelength band have conventionally been studied for use as an optical transmission line. Here, the dispersion slope refers to the gradient of a graph indicating the wavelength dependence of dispersion.
As a result of studies concerning the above-mentioned conventional techniques, the inventors have found the following problems.
Namely, as compared with typical single-mode optical fibers having a zero-dispersion wavelength near a wavelength of 1.3 xcexcm, the wavelength band dispersion-flattened optical fibers tend to have a greater optical energy per unit cross-sectional area since their effective area is smaller, though yielding a smaller dispersion in the 1.5-xcexcm wavelength band. It means that nonlinear optical phenomena (four-wave mixing in particular) are relatively easily occur in the dispersion-flattened optical fibers. On the other hand, in an optical transmission system employing a dispersion-flattened optical fiber as an optical transmission line, it is necessary to reduce the power of signal light sent out from its transmitting station or repeater station, whereby the repeater spacing is inevitably shortened. It means that the number of stations to be installed would increase, whereby the optical transmission system to be realized becomes more expensive.
The optical transmission system employing a single-mode optical fiber as its optical transmission line and comprising a dispersion-compensating module for compensating for the dispersion of the optical transmission line is designed such that both the dispersion and dispersion slope are substantially zero in the 1.5-xcexcm wavelength band. Also, as the dispersion-compensating module, one having a negative dispersion in the 1.5-xcexcm wavelength band is employed. Since the dispersion-compensating module is desired to have a smaller size, the dispersion-compensating fiber to be employed in the dispersion-compensating module is required to be designed to yield large absolute values of both dispersion and dispersion slope so that the dispersion of the whole optical transmission line can be compensated for by a short length of the dispersion-compensating fiber. Therefore, the effective area of the dispersion-compensating optical fiber is very small. Since the dispersion-compensating fiber is wound like a coil having a diameter on the order of 50 to 100 mm, how to reduce its bending loss is an important technical issue in such a dispersion-compensating fiber. Here, since the dispersion-compensating fiber employed in the dispersion-compensating module has a large dispersion value, it cannot be utilized as a main line though it constitutes a part of the optical transmission line.
For overcoming the problems such as those mentioned above, it is an object of the present invention to provide an optical transmission line which enables WDM communications in a wide signal light wavelength band (1.5-xcexcm wavelength band) centered at a wavelength of 1.55 xcexcm and effectively restrains nonlinear optical phenomena from occurring, and an optical fiber constituting a part of the optical transmission line. In the following, the optical fiber according to the present invention will be referred to as a xe2x80x9cdispersion-equalizing optical fiberxe2x80x9d, the use of which reduces dispersion and dispersion slope in the optical transmission line as a whole.
The dispersion-equalizing optical fiber according to the present invention is employed in a part of an optical transmission line which is a transmission medium for WDM communications between stations such as between a transmitting station and a receiving station, between repeater stations, between the transmitting station and a repeater station, between a repeater station and the receiving station, or the like. This dispersion-equalizing optical fiber is an optical fiber for reducing deviations in dispersion among wavelengths in its signal light wavelength band; and comprises a core region extending along a predetermined axis and a cladding region disposed on the outer periphery of the core region. The above-mentioned cladding region may comprise a depressed cladding structure constituted, at least, by an inner cladding, disposed on the outer periphery of the core region, having a refractive index lower than that of the core region; and an outer cladding, disposed on the outer periphery of the inner cladding, having a refractive index higher than that of the inner cladding.
In particular, the dispersion-equalizing optical fiber according to the present invention has a dispersion D (unit: ps/nm/km) and a dispersion slope S (unit: ps/nm2/km) satisfying the following conditions:
xe2x88x9283xe2x89xa6Dxe2x89xa6xe2x88x9218
0.0050xc3x97Dxe2x89xa6Sxe2x89xa60.0025xc3x97D
with respect to light having a wavelength of 1.55 xcexcm.
Further, as characteristics with respect to light having a wavelength of 1.55 xcexcm, this dispersion-equalizing optical fiber has an effective area of 15 xcexcm2 or more, preferably 17 xcexcm2 or more, further preferably 19 xcexcm2 or more. Here, as disclosed in Japanese Patent Application Laid-Open No. HEI 8-248251 (EP 0 724 171 A2), the effective area Aeff is given by the following expression (1);                               A          eff                =                  2          ⁢                                                    π                ⁡                                  (                                                            ∫                      0                      ∞                                        ⁢                                                                  E                        2                                            ⁢                      r                      ⁢                                              ⅆ                        r                                                                              )                                            2                        /                          (                                                ∫                  0                  ∞                                ⁢                                                      E                    4                                    ⁢                  r                  ⁢                                      ⅆ                    r                                                              )                                                          (        1        )            
where E is the electric field accompanying the propagating light, and r is the radial distance from the core center.
Since the dispersion D and dispersion slope S at the wavelength of 1.55 xcexcm satisfy the above-mentioned conditions in this dispersion-equalizing optical fiber, when the ratio between the length of the dispersion-equalizing optical fiber and the length of a single-mode optical fiber having a zero-dispersion wavelength in the 1.3-xcexcm wavelength band is appropriately adjusted, the respective absolute values of dispersion and dispersion slope in the whole optical transmission line constituted by the dispersion-equalizing optical fiber and single-mode optical fiber can be minimized (wavelength dependence can be reduced). Since the dispersion-equalizing optical fiber has an effective area of 15 xcexcm2 or more preferably 17 xcexcm2 or more, it effectively restrains nonlinear optical phenomena from occurring when disposed downstream from the single-mode optical fiber. For securing a higher transmission quality, it is preferable for the dispersion-equalizing optical fiber to have an effective area of 19 xcexcm2 or more. As a consequence of such a configuration, the bending loss of the dispersion-equalizing optical fiber becomes 50 dB/m or less, preferably 10 dB/m or less with respect to light having a wavelength of 1.55 xcexcm when wound at a diameter of 20 mm.
The dispersion-equalizing optical fiber according to the present invention may have a refractive index profile of a depressed cladding structure realized by the above-mentioned core region and a cladding region comprising an inner cladding and an outer cladding. Consequently, the respective values of dispersion, dispersion slope, and effective area can easily be designed so as to satisfy the above-mentioned conditions and ranges. Also, the dispersion-equalizing optical fiber may further comprise an intermediate cladding disposed between the inner cladding and outer cladding. The intermediate cladding has a refractive index higher than that of the outer cladding and lower than that of the core region, and may be disposed in direct contact with the inner cladding or disposed on the outer periphery of the inner cladding by way of another intermediate cladding having a refractive index lower than that of the former intermediate cladding. In any of these configurations, a refractive index profile of a depressed cladding structure is realized.
In the dispersion-equalizing optical fiber according to the present invention, the core region has a relative refractive index difference of 0.72% or more but 1.8% or less, preferably 0.9% or more but 1.6% or less, with respect to the cladding region or the outer cladding (in the case of a depressed cladding structure). In such a case, the respective values of dispersion, dispersion slope, and effective area can also easily be designed so as to satisfy the above-mentioned conditions and ranges. Therefore, with respect to light having a wavelength of 1.55 xcexcm, the bending loss at a diameter of 20 mm can easily be made 50 dB/m or less, preferably 10 dB/m or less. For yielding further preferable transmission characteristics, the polarization mode dispersion of the dispersion-equalizing optical fiber is 0.15 psxc2x7kmxe2x88x921/2 or less with respect to light having a wavelength of 1.55 xcexcm.
The bending loss, which becomes a limiting factor for making a module, can be reduced not only by enhancing the effective area as mentioned above, but also by adjusting the fiber diameter, the outside diameter of a coating layer covering the dispersion-equalizing optical fiber, and the like. Specifically, as the fiber diameter increases from a standard of 125 xcexcm, the effect of reducing the bending loss enhances. For securing the flexibility of the dispersion-equalizing optical fiber, however, the upper limit of the fiber diameter is preferably 200 xcexcm or less. On the other hand, when adjusting the outside diameter of the coating layer disposed on the outer periphery of the dispersion-equalizing optical fiber, the effect of reducing the bending loss can be obtained if the outside diameter of the coating layer is 235 xcexcm or more. For securing a flexibility sufficient for constructing a module, the outside diameter of the coating layer is preferably 415 xcexcm or less. While a desirable reducing effect can be obtained when one of the fiber diameter and the outside diameter of the coating layer is adjusted, similar effects can also be obtained when these two adjusting methods are combined together. Namely, when reducing the bending loss by increasing the outside diameter of the coating layer, a desirable reducing effect can be obtained even if the fiber diameter is decreased, whereby it will be sufficient if the fiber diameter is 115 xcexcm or more.
The optical transmission line according to the present invention is a transmission medium disposed between stations for transmitting/receiving data, such as a transmitting station, repeater stations, and a receiving station; and comprises, as viewed in the propagating direction of WDM signals, a single-mode optical fiber, disposed on the upstream side, having a zero-dispersion wavelength near a wavelength of 1.3 xcexcm, specifically within the range from 1.25 xcexcm to 1.45 xcexcm, and the above-mentioned dispersion-equalizing optical fiber disposed on the downstream side. In such an optical transmission line, when the length of the single-mode optical fiber and the length of the dispersion-equalizing optical fiber are set so as to have an appropriate ratio therebetween, the respective absolute values of the dispersion and dispersion slope in the whole optical transmission line can be minimized. Here, since the line comprising the dispersion-equalizing optical fiber and the single-mode optical fiber allows to be connected to a station by way of another single-mode optical fiber such as a dispersion-shifted optical fiber, the total length m of the dispersion-equalizing optical fiber and single-mode optical fiber preferably satisfies the following condition:
0.9xc3x97L less than mxe2x89xa6L
where L is the length of the optical transmission line, i.e., the distance between stations between which the optical transmission line is installed.
In the above-mentioned optical transmission line, since the dispersion-equalizing optical fiber (having an effective area of 15 xcexcm2 or more) is disposed downstream from the single-mode optical fiber (e.g., an optical fiber whose core is doped with Ge element), nonlinear optical phenomena are restrained from occurring. In particular, when the transmission loss of this single-mode optical fiber is 3.3 dB or more with respect to light having a wavelength of 1.55 xcexcm, if the signal light sent out from the transmitting station has such a power that nonlinear optical phenomena do not occur (or do not become problematic if any) in the single-mode optical fiber, then the nonlinear optical phenomena are also sufficiently restrained from occurring in the dispersion-equalizing optical fiber located on the downstream side.
Since the dispersion of the dispersion-equalizing optical fiber at a wavelength of 1.55 xcexcm is xe2x88x9283 ps/nm/km or more but xe2x88x9218 ps/nm/km or less, and the dispersion of the single-mode optical fiber at the wavelength of 1.55 xcexcm is 17 ps/nm/km, the ratio of the length of the single-mode optical fiber to the length of the dispersion-equalizing optical fiber is on the order of 1:1 to 1:4.9. When such an optical transmission line is employed as the transmission line between individual repeaters in a submarine cable, in view of the fact that one span (repeater spacing) of the submarine cable is about 50 km in general, it is necessary for the above-mentioned single-mode optical fiber to have a length of less than 42 km. Further, for effectively suppressing the nonlinear optical phenomena, the upper limit of the ratio of length occupied by the single-mode optical fiber in the optical transmission line is about 73% (36.5 km). At this time, letting the transmission loss of the single-mode optical fiber whose core is doped with Ge element be 0.195 dB/km, the upper limit of the total transmission loss in the single-mode optical fiber is preferably 7.1 dB or less.
On the other hand, the single-mode optical fiber (having a zero-dispersion wavelength near 1.3 xcexcm) constituting a part of the above-mentioned optical transmission line may be a single-mode optical fiber constituted by a core and a cladding, in which the cladding in particular is doped with F element (whereas the core is made of pure silica). This optical transmission line is favorable not only in that it can minimize the respective absolute values of total dispersion and dispersion slope, thus restraining nonlinear optical phenomena from occurring, but also in that transmission loss and splice loss are small. In particular, when the transmission loss of this F-doped single-mode optical fiber is 3.0 dB or more with respect to light having a wavelength of 1.55 xcexcm, if the signal light sent out from the transmitting station has such a power that nonlinear optical phenomena do not occur (or do not become problematic if any), then the nonlinear optical phenomena are sufficiently restrained from occurring in the dispersion-equalizing optical fiber located on the downstream side as well. When its application to a submarine cable having one span of about 50 km is taken into consideration, the single-mode optical fiber having an F-doped cladding is also required to have a length of less than 42 km. Also, for effectively suppressing the nonlinear optical phenomena, the upper limit of the ratio of length of the single-mode optical fiber in the optical transmission line is preferably about 73% (36.5 km); and, letting the transmission loss of the F-doped single-mode optical fiber be 0.175 dB/km, the total transmission loss in the F-doped single-mode optical fiber is preferably 6.4 dB or less.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.